moespeeds

I've made a few custom exhaust systems for motorcycles, but never for a car. I'm kicking around the idea of making my own system for my C5 when I take out the cats. The car is track only, so I'm not concerned about inspection. Right now the exhaust is stock. Do I need to do anything special to create backpressure? I know on motorcycles this is an issue. At this point I'm 100% ignorant when it comes to cars.

AU N EGL

Nope

the pipe itself is enough

Lothar34

I have yet to hear any reasonable explanation why you would want resistance on the exhaust stroke.

If there was a benefit to backpressure, why don't these people work it in?:

Oldvetter

To gain the most benefit, you may need to do a tune. People usually get a tune when they install headers.

wcsinx

Probably because they don't use those to get groceries.

hotwheels57

And they're operating at peak rpm for only 1000-1320 ft at a time...

R CRYSIS

Supercharged cars don't need back pressure like a N.A. car. That's why dragsters run Zoomy headers. I have found that a little back pressure in a well made exhaust actually helps in throttle response and low end torque. If you look at a sprint cup car you will see that they run an exhaust system through an X pipe and out the side but with no mufflers. I race motorcycles also and I have seen just what you are talking about when guys take off there muffler and the bike falls on it's face. It is also a 4 cycle engine. IMHO I think if you ran pipes out the back of the car even with no muffler it would be enough. That's pretty much what some of the systems out there already are

moespeeds

Yeah what I was planning was to eliminate cat, then make an x pipe, and delete the mufflers off the stock pipe. What's with the split to 4 tips is it Just for show? Can I run two tips?

Bakerking31

4 tips is just for show, you can do 2 and it will just look different.

That setup will have plenty of back pressure so its not a worry, as long as you have an X or H pipe you will be fine.

Lothar34

No, but that doesn't really answer the question either.

For whichever piston is on the exhaust stroke, there is another somewhere else on the power stroke. It's already overcoming the resistance of the friction, the drivetrain, and maybe intake vacuum. Why would adding resistance via exhaust pressure help the engine?

R CRYSIS

I have had this explained to me before by a friend of mine who builds some of the fastest motors around for pro stock(I actually met Warren Johnson in his shop!!!) but I am not going to pretend to be an expert. It has something to do with the scavenging effect that the cylinders have one each other if the exhaust is done properly with an X pipe and the velocity it has coming out of the port. I have heard about a test where Flowmaster actually got more horsepower and torque with there exhaust with an X pipe out of a big block chevy in a Chevelle than they could get with open headers. I think it also is different depending on the engine and the application.

Lothar34

Yeah scavenging is great, but that's a way to take advantage of the resistance in long pipes. If you don't have the resistance though...

moespeeds

When I ditch the cats will I need O2 sims?

I'm no expert but I know from direct experience on motorcycles that if you don't run some kind of back pressure on short pipes you noticeably lose low end and it's a raging bitch to tune. That's also on a carburated engine using 1930's farm tractor technology...

Lothar34

If you're just racing then you don't need sims, but the CEL will keep coming on.

If you're going to tune it then you can set the rear O2s to report ready all the time and not have to deal with the CEL.

wcsinx

I'll spell it out for you.

Streetability

Yes, you will get more peak power, but the engine's low and mid range characteristics will suffer in every way.

Then by all means cut the exhaust system off your C5 and see how that works out for you.

moespeeds

Can I just cut open those mufflers and gut them, or should I cut them out and run them straight? Gutting them would save me the time of welding on new tips and lining it all up.

wcsinx

Gutted mufflers sound like azz IMHO. I would just run straights. Just be sure you get a proper x-pipe if you do because you'll sound like a motorboat at idle otherwise.

Lothar34

It would sound like ***, and since it's a passenger car the exhaust needs to exit behind the cabin.

Still if I was doing a complete custom exhaust, I wouldn't try to work extra backpressure into my design.

Dave68

Exactly. This is why you should NOT have 4 inch diameter pipes unless your engine output is high enough.

I recall many years ago, Lingenfelter found that combined with a twin turbo monster engine, a set of Borla Stingers (cat-back) gained him a significant amount of horsepower over combining the same engine with stock exhaust. I believe the Borla gain was something like 100 HP.
Of course, if you simply throw on a set of Stingers with a stock engine, your gain will be minimal because a stock engine can't fully take advantage of a more open exhaust. A less -restrictive intake will benefit slightly.

lionelhutz

Geeze;

Backpressure - dumb and useless explanation perpetuated by word of mouth and the internet.

Flow Velocity - what you really want, especially close to the heads.

Flow velocity is what's set up in headers to cause scavanging as already mentioned. It just happens that you will get some resistance to flow due to having a certain pipe diameter and flow rate. Ideally, you don't want it.

You NEVER want to intentionally restrict the exhaust on your car.


Relatively large diameter and short pipes = poor flow velocity